1. Field of the Invention
The present invention relates to a Liquid-Crystal Display (LCD) device and more particularly, to an LCD device using the horizontal electric-field running approximately parallel to a pair of glass substrates in a liquid crystal layer, which prevents the display quality from degrading due to static electricity charged on one or both of the substrates and so on.
2. Description of the Prior Art
In recent years, LCD devices have been extensively used as display terminals for electronic equipment, such as personal computers and word processors. This is because LCD devices have advantages that they dissipate low electric power, they are compact and light-weight, and they are operable at a low supply voltage.
LCD devices have various driving methods such as the static driving, multiplex driving, and active matrix driving methods, all of which have been used in practice. Nowadays, the active matrix driving method has formed its main stream.
LCDs using the active matrix driving method, i.e., active-matrix LCDs, are divided into two groups by the difference in electrode structure; the vertical electric-field and horizontal electric-field types.
With the vertical electric-field type LCDs, a liquid crystal is confined in the space between first and second glass substrates coupled together to be opposite and parallel. Display electrodes for forming pixels and Thin-Film Transistors (TFTs) for selecting the pixels to be driven are formed on the first substrate. A common electrode is formed on the second substrate to be opposite to the display electrodes. A driving voltage is selectively applied across desired ones of the display electrodes and the common electrode, thereby applying a vertical electric-field to the liquid crystal. Thus, the orientation of the molecules of the liquid crystal is changed continuously from the horizontal direction to the vertical one, thereby controlling or modulating the transmitted light through the liquid crystal layer.
The vertical electric-field type LCDs have the following disadvantage.
Specifically, when a user look at the display area of the LCD device of this type obliquely, he tends to see sudden change in brightness or inversion of light and shade. In other words, the LCDs of this type have a bad viewing-angle characteristic. This is because the molecules of the liquid crystal that have been aligned vertically with respect to the substrates have different apparent lengths for the user.
On the other hand, with the horizontal electric-field type LCDs, a liquid crystal is confined in the space between first and second glass substrates coupled together to be opposite and parallel. This structure is the same as that of the vertical electric-field type ones. Unlike the vertical electric-field type ones, display electrodes for forming pixels, TFTs for selecting the pixels to be driven, and reference electrodes are formed on the first substrate. No electrodes are formed on the second substrate. A driving voltage is selectively applied across desired ones of the display electrodes and those of the reference electrodes, thereby applying a horizontal electric-field to the liquid crystal. Thus, the orientation of the molecules of the liquid crystal is changed continuously from the vertical direction to the horizontal one, thereby controlling or modulating the transmitted light through the liquid crystal layer.
With the horizontal electric-field type LCD devices, unlike the vertical electric-field type ones, the above-described disadvantage of the bad viewing-angle characteristic can be improved. This is because the molecules of the liquid crystal that have been aligned horizontally with respect to the substrates have a small difference in apparent length. However, the LCD devices of this type might have a problem that the display quality tends to degrade. For example, if the LCD device is of the normally black mode, the display area tends to be partially or entirely turned white. If the LCD device is of the normally white mode, it tends to be partially or entirely turned black. The problem is caused by the following reason.
Specifically, the second substrate (to which no electric-field is directly applied) might be electrified due to static electricity or some unintended electric-field, which may be applied from the outside of the LCD device. In this case, electric charges are stored on the second substrate and therefore, they create unwanted vertical electric-field with respect to the first and second substrates in the liquid crystal layer. Thus, the orientation of the molecules of the liquid crystal tends to be changed toward the vertical direction due to the unwanted vertical electric-field, resulting in degradation of the display quality.
To solve the above-described problem of display quality degradation in the horizontal electric-field type LCD devices, various solutions have been developed and reported. An example of the solutions is disclosed in the Japanese Non-Examined Patent Publication No. 9-105918 published in April 1997.
FIGS. 1 and 2 show the structures of the prior-art horizontal electric-field type LCD devices disclosed in the Publication No. 9-105918.
In FIG. 1, a glass substrate 101 and another glass substrate 102 are coupled together to be opposite and parallel, forming a space therebetween. The space is sealed by a sealing member 109 located along the whole periphery of the substrate 101. Liquid crystal LC is confined in the sealed space, forming a liquid-crystal layer. Although not shown in FIG. 1, display electrodes, reference electrodes, and TFTs are formed on the inner surface of the substrate 102.
A transparent conductive layer 103 is additionally formed on the outer surface of the substrate 101. A metal frame 104, which surrounds entirely the substrate 101 as a casing, is mechanically and electrically connected to the conductive layer 103 at the periphery of the substrate 101 by way of a conductive rubber layer 105. Thus, even if electric charges are stored on the substrate 101, they are quickly discharged toward outside through the conductive layer 103, the conductive rubber layer 105, and the frame 104, thereby preventing the display quality from degrading due to any unwanted vertical electric-field existing in the liquid crystal LC.
The structure of the prior-art LCD device shown in FIG. 2 is the same as that shown in FIG. 1, except that the transparent conductive layer 103 is electrically connected to a grounding terminal 106 formed on the inner surface of the substrate 102 by way of a cable 107.
The above-identified Publication No. 9-105981 further discloses that a conductive paste or a conductive metal tape may be used instead of the cable 107. The conductive metal tape is formed by a metal tape and an adherent material containing a conductive substance.
The prior-art LCD devices shown in FIGS. 1 and 2 have problems explained below.
With the prior-art LCD device shown in FIG. 1, the conductive rubber layer 105 is used to electrically connect the conductive layer 103 to the metal frame 104 and therefore, the thickness of the LCD device becomes larger. In recent years, there has been the strong need that the thickness of LCD devices is set to be possibly smaller. From this point of view, the structure of FIG. 1 is not preferred.
Also, when the metal frame 104 is connected to the substrate 101, the rubber layer 105 needs to be strongly pressed onto the substrate 101 in order to stabilize the contact resistance of the rubber layer 105. At this stage, a counter force to the pressing force is applied to the frame 104 and as a result, some deformation tends to occur in the frame 104. Thus, the pressing force is unable to be set to be as high as desired.
Furthermore, according to the result of the inventor""s test, the following problem was found.
Specifically, when a pressure equal to or greater than a specific value is applied to the substrate 101 in the adhesion process of the frame 104, there arises a phenomenon that the display area might be turned to be white or black at a corresponding location to the pressure-applied part in spite of no electric-field being applied to the substrate 101. If the LCD device is of the normally black mode, the display area tends to be turned white. If the LCD device is of the normally white mode, the display area tends to be turned black.
As a result, because of the two above-described reasons, the pressing force applied to the substrate 101 in the adhesion process of the frame 104 needs to be set to be equal to or lower than a specific value.
Furthermore, with the LCD device shown in FIG. 2, since the cable 107 needs to be used, there is a problem that a complicated process is required for interconnecting the conductive layer 103 with the grounding terminal 106 by way of the cable 107.
There arises a problem that a similar complicated process is needed if the conductive paste or conductive metal tape is used instead of the cable 107. Also, in this case, the following problem will occur.
Specifically, when the conductive paste is used, it tends to flow toward the substrate 102 due to the gravity in the interconnection process of the conductive layer 103 and the grounding terminal 106. Thus, the coated paste is difficult to have a desired thickness or cross-sectional area. In particular, the thickness of the coated paste tends to be decreased at the corner 108 of the conductive layer 103. As a result, the electric resistance of the coated paste tends to fluctuate in a wide range, which causes a possibility that the unwanted electric charges on the substrate 101 are unable to be fully removed.
When the conductive metal tape is used, there is a possibility that a satisfactory adhesion strength of the tape is unable to be produced. This is because the tape contains a conductive substance in the adherent material and therefore, the surface area effective to adhesion of the tape is decreased.
Accordingly, an object of the present invention is to provide a horizontal electric-field type LCD device that prevents the display quality from degrading due to unwanted electric field with a thin and simple structure.
Another object of the present invention is to provide a horizontal electric-field type LCD device that requires no complicated process for electrically interconnecting a protecting conductive layer on a substrate with a grounding electrode on another substrate.
Still another object of the present invention is to provide a horizontal electric-field type LCD device that provides proper pressing forces for a protecting conductive layer on a substrate and a grounding electrode on another substrate.
A further object of the present invention is to provide a horizontal electric-field type LCD device that makes it easy to replace a driver IC (Integrated Circuit).
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
A horizontal electric-field type LCD device according to the present invention is comprised of:
(a) a first transparent substrate;
(b) a protecting conductive layer formed on an outer surface of the first substrate;
(c) a second transparent substrate coupled with the first substrate to form a space between inner surfaces of the first and second substrates;
the second substrate having an extended part that extend from the periphery of the first substrate;
(d) a grounding electrode formed on the inner surface of the extended part of the second substrate;
(e) a liquid crystal confined in the space formed between the inner surfaces of the first and second substrates; and
(f) a conductive, elastic connection member for electrically interconnecting the conductive layer on the first substrate with the grounding electrode on the second substrate;
the connection member having a base part engaged with an outer surface of the second substrate, a first spring part engaged with the conductive layer on the first substrate, and a second spring part engaged with the grounding electrode on the second substrate.
With the horizontal electric-field type LCD device according to the present invention, the protecting conductive layer on the outer surface of the first substrate and the grounding electrode on the inner surface of the extended part of the second substrate are electrically interconnected with each other by way of the conductive, elastic connection member. Thus, even if electrification occurs on the first substrate to create electric charges thereon, these electric charges flow quickly to the grounding electrode through the connection member. As a result, the display quality does not degrade due to the unwanted electric field generated by the electric charges on the first substrate.
Also, the connection member has the base part engaged with the outer surface of the second substrate, the first spring part engaged with the conductive layer on the outer surface of the first substrate, and the second spring part engaged with the grounding electrode on the inner surface of the second substrate. Therefore, the structure of the LCD device can be made thin and simple. At the same time as this, the member can be easily attached to the first and second substrates and easily detached therefrom, which means that a driver IC can be easily replaced.
Furthermore, since the cable and the conductive rubber layer are not used, no complicated process is required for electrically interconnecting the conductive layer with the grounding electrode.
Moreover, elastic deformation occurs independently in the first and second spring parts of the connection member. Therefore, proper pressing forces can be provided for the respective engagements of the first and second spring parts, which stabilizes these engagements.
In a preferred embodiment of the device according to the present invention, the first spring part and the base part of the connection member form an approximately U-shaped cross section, and the second spring part and the base part of the connection member form an approximately U-shaped cross section. In this embodiment, there is an additional advantage that the connection member can be formed by a thin metal plate or sheet while the electric resistance of the member is kept satisfactorily low. This means that the LCD device can be compact and that stable electrical connection between the protecting conductive layer and the grounding electrode is ensured.
In another preferred embodiment of the device according to the present invention, the connection member is made of a material selected from the group consisting of stainless steel, phosphor bronze, and beryllium copper. In this embodiment, there is an additional advantage that the connection member can be mechanically processed easily and has a good durability.
In still another preferred embodiment of the device according to the present invention, the base part of the connection member is engaged with the outer surface of the second substrate by way of a rubber sheet. In this embodiment, there is an additional advantage that the mechanical engagement of the base part of the connection member with the second substrate is reinforced, thereby decreasing the danger that the engagement between the member and the second substrate is released due to applied vibration or shock.
It is preferred that the rubber sheet is adhered to the base part of the connection member. In this case, the mechanical engagement of the base part with the second substrate is further reinforced.
In a further preferred embodiment of the device according to the present invention, the first spring part of the connection member is designed to apply a pressure of approximately 30 gw/cm2 (=2941.995 Pa) or lower to the protecting conductive layer. In this embodiment, there is an additional advantage that the possibility that the display area is unintentionally turned to white or black can be prevented, because the applied pressure to the first substrate is limited. This leads to further improvement of the display quality.